This invention relates generally to tubes used in heat exchangers for transferring heat between a fluid inside the tube and a fluid outside the tube. More particularly, the invention relates to a heat transfer tube having an internal surface that is capable of enhancing the heat transfer performance of the tube. Heat exchangers of air conditioning and refrigeration (AC&R) or similar systems contain such tubes.
Designers of heat transfer tubes have long recognized that the heat transfer performance of a tube having surface enhancements is superior to a smooth walled tube. Manufacturers have applied a wide variety of surface enhancements to both internal and external tube surfaces including ribs, fins, coatings and inserts, to name just a few. Common to nearly all enhancement designs is an attempt to increase the heat transfer area of the tube. Most designs also attempt to encourage turbulence in the fluid flowing through or over the tube in order to promote fluid mixing and break up the boundary layer at the surface of the tube.
A large percentage of AC&R, as well as engine cooling, heat exchangers are of the plate fin and tube type. In such heat exchangers, plate fins affixed to the exterior of the tubes are the tube external enhancements. The heat transfer tubes frequently also have internal heat transfer enhancements on the interior wall of the tube.
Many prior art internal surface enhancements in metal heat transfer tubes are ribs formed by working the tube wall in some way. Such ribs frequently run in a helical pattern around the tube surface. This is a prevalent configuration because helical rib patterns are usually relatively easier to form than other types of rib patterns. Thorough mixing, turbulent flow and the greatest possible internal heat transfer surface area are desirable to promote heat transfer effectiveness. However, high rib heights and rib helix angles can result in flow resistance that is so high that flow pressure losses become unacceptable. Excessive pressure losses require excessive pumping power and an overall degradation of system efficiency. Tube wall strength and integrity are also considerations in how to configure an internal surface enhancement.
As is implicit in their names, the fluid flowing through a condenser undergoes a phase change from gas to liquid and the fluid flowing through an evaporator changes phase from a liquid to a gas. Heat exchangers of both types are needed in vapor compression AC&R systems. In order to simplify acquisition and stocking as well as to reduce costs of manufacturing, it is desirable that the same type of tubing be used to in all the heat exchangers of a system. But heat transfer tubing that is optimized for use in one application frequently does not perform as well when used in the other application. To obtain maximum performance in a given system under these circumstances, it would be necessary to use two types of tubing, one for each functional application. But there is at least one type of AC&R system where a given heat exchanger must perform both functions, i.e. a reversible vapor compression or heat pump type air conditioning system. It is not possible to optimize a given heat exchanger for a single function in such a system and the heat transfer tube selected must be able to perform both functions well.
In a significant proportion of the total length of the tubing in a typical plate fin and tube AC&R heat exchanger, the flow of refrigerant flow is mixed, i.e., the refrigerant exists in both liquid and vapor states. Because of the variation in density, the liquid refrigerant flows along the bottom of the tube and the vaporous refrigerant flows along the top. Heat transfer performance of the tube is improved if there is improved intermixing between the fluids in the two states, e.g. by promoting drainage of liquid from the upper region of the tube in a condensing application or encouraging liquid to flow up the tube inner wall by capillary action in an evaporating application.
To obtain improved heat transfer performance as well as to simplify manufacturing and reduce costs, what is needed is an heat transfer tube that has a heat transfer enhancing interior surface that is simple to produce, has at least an acceptably low resistance to fluid flow and can perform well in both condensing and evaporating applications. The interior heat transfer surface must be readily and inexpensively manufactured.